Resistive layer track potentiometers



Jan. 12, 1965 us 3,165,714

RESISTIVE LAYER TRACK POTENTIOMETERS Filed Aug. 6, 1962 United States Patent 3,165,714 RESISTIVE LAYER TRACK POTENTIOMETERS Bertranrl'Alain Dreyfus, Sevres, France, assiguor to Societe dElectronique et dAutomatisme, Courbevoie, Seine, France Filed Aug. 6, 1962, Ser. No. 215,489 Claims priority, appiication France, Sept. 4, 1961, 872,409, Patent 1,306,701 7 Claims. (Cl. 338162) The present invention concerns improvements in or relating to potentiometers the track of which is composed of a resistive layer of a complex of oxides and/ or nitrides comprising a coating on an insulating glass or ceramic base plate. Most often such tracks are annular bands on a disc-shaped base plate. Such layers may be obtained, as known, by spraying an acidified solution of halogens of elements the oxides and/ or nitrides of which will be the bases of the layer onto the insulating base plate. The base plate is preferably heated up to the pyrolysis temperature of deposition of the said oxides and/or nitrides according to the atmosphere in which the spraying is conducted. Usually the best material for such tracks is tin oxide to which certain impurities are added for adjusting the resistivity of the layer. Nitridation, if any, is made after the deposition of such an oxide complex.

An important problem in these potentiometers is the brushes which engage the resistive track. The problem is to obtain an efiicient and positive contact between the track and the brush because the contact resistance must be satisfactorily low without a high contact pressure which would damage the film layer of the track and accelerate the wear of both the track and the brush. Further, said contact resistance must be constant in order to preserve the uniformity ofthe electrical connection across the complete track, as well as from repeated passage of the brush along the track. For such appli cations of these potentiometers as in servo-mechanisms, the mechanical inertia of the arm carrying the brush must be as small as possible.

Accordingly, it is the principal object of the present invention to provide a new and useful potentiometer arrangement which ensures superior operating conditions and longer life together with uniform contact between the wiper arm and the resistive track with low contact pressure.

According to the invention, a potentiometer having a film track made of a complex of oxides and/ or nitrides is further provided with a film deposit of a metal or metal alloy along the path followed by the brush along said track. The brush is mounted on a small inertia arm with a low pressure contact on said metallic deposit. The electrical resistance of said deposit, in parallel with that of the track with respect to the brush, is made relatively high with respect to the electrical resistance of the track in order that said electrical resistance of said deposit does not disturb the electrical characteristics of the complete potentiometer.

Such an additional metallic deposit defining the path for the brush may be made either by a vacuum or an electrolytic process. It is to be considered as a complex metal oxide intimately bound to the material of the film track proper for providing the intrinsic properties of strong adherence, relatively high electrical resistance and mechanical endurance against the friction of the brush which have been displayed in practice by this arrangement. This consequently gives a stable and uniform electrical contact both along the track and during operation of the potentiometers.

3,165,714 Patented Jan. .12, 1965 In the drawing:

FIGURE 1 is a plan View of a preferred embodiment of the invention.

FIGURE 2 is an elevational view of the potentiometer of FIGURE 1 in axial section through the shaft and the brush arm.

In the figures illustrating schematically the arrangement of the invention as applied to a circular potentiometer, a disc-shaped member 1, refractory glass or ceramic, is coated by a film layer 2 of a complex of oxides based on the preponderance of tin oxide therein. The track 2 ends in two terminals 5 and 4, leaving a small angular sector 10 bare on the disc 1. On said terminals of low intrinsic resistance, are soldered the supply leads 6 and 7.

The disc is perforated at its center portion and an annular band 9 around said hole is free of oxide coating. In said hole is mounted the carrier 13 of the arm 12 which carries the brush 11 at its end. Under said brush, on the track, 3 is the metallic deposit defining the path along which the brush will move when the carrier 13 rotates.

The metallic deposit 3 may be for instance of chromium or nickel-chromium evaporated under vacuum. It also may be made of electrolytically deposited rhodium. The bonding of chromium or nickel-chromium is quite good as is that of rhodium but the latter, though being of more delicate deposition, is superior from the standpoint of resistance to oxidation and mechanical resistance to abrasion.

Even better quality is obtained from electrolytic deposition of a tin-nickel alloy on a track base of tin oxide since both the tin-nickel alloy and the tin oxide base have a tetrahedric structure. Such identity of the molecular structure ensures the formation of the deposit 3 under the actual form of a metal oxide complex intimately bonded to the oxide track 2. Further the tinnickel alloy is unoxydable even in strenuous conditions and it is very corrosion resistant to a wide variety of corrosive media.

During the deposition of the path 3 on the track, the portions of the track not to be coated are protected by varnish, plastic material or silk screen or the like.

With a potentiometer arrangement according to the invention, the slider contact resistance obtained was 2-ohms for the tin-nickel alloy with a contact pressure of 20 grams. A rhodium deposit gave a contact resistance of about ten ohms for a contact pressure of some hundreds of grams, such values being similar for chromium and nickel-chromium deposits.

Other metals may be used and, obviously, the invention may be applied to linear motion potentiometers as well as circular motion ones.

I claim:

1. A potentiometer comprising in combination: an insulating base member; a resistive film track formed of a metallic compound carried by said base member; a metallic brush track which is narrow and substantially of brush width formed over said resistive film track and extending continuously from one end to the other of the path of brush travel, said brush track being of substantially higher resistance than said film track; and a low-pressure brush mounted to travel along and in contact with said brush track.

2. A potentiometer as defined by claim 1 in which said resistive film track is a compound selected from the group consisting of metallic oxides and metallic nitrides.

3. A potentiometer as defined by claim 1 in which said brush track comprises vacuum deposited chromium.

4. A potentiometer as defined by claim 1 in which rhodium.

5. A potentiometer as defined by claim 1 in which said brush track comprises an electrolytically deposited tin-nickel alloy.

6. A potentiometer as defined by claim 1 in which said resistive film track comprises a complex oxide film, the main component ofwhich is'tin-oxide, and in which said brush track comprises atin-nickel alloy bonded to said resistive film track.

7. A potentiometer as defined "by claim 1 in which said brush track comprises a vacuum deposited nickelchromium alloy.

References Cited in the file of this patent UNITED STATES PATENTS Ruben May 2, 1933 Ruben Aug. 21, 1934 Stoekle June 25, 1935 Schleuning Sept. 7, 1954 Kohring Aug. 20, 1957 Moore Mar. 18, 1958 Iwerson et a1 Apr. 10, 1962 FOREIGN PATENTS Germany Aug. 14, 1943 

1. A POTENTIOMETER COMPRISING IN COMBINATION: AN INSULATING BASE MEMBER; A RESISTIVE FILM TRACK FORMED OF A METALLIC COMPOUND CARRIED BY SAID BASE MEMBER; A METALLIC BRUSH TRACK WHICH IS NARROW AND SUBSTANTIALLY OF BRUSH WIDTH FORMED OVER SAID RESISTIVE FILM TRACK AND EXTENDING CONTINUOUSLY FROM ONE END OF THE OTHER OF THE PATH OF BRUSH TRAVEL, SAID BRUSH TRACK BEING OF SUBSTANTIALLY HIGHER RESISTANCE THAN SAID FILM TRACK; AND A LOW-PRESSURE BRUSH MOUNTED TO TRAVEL ALONG AND IN CONTACT WITH SAID BRUSH TRACK. 